Body area networks have received considerable attention in the last five years, driven by the fascinating promise of a future where carefully engineered miniaturized biomedical devices implanted, ingested or worn by humans can be wirelessly internetworked to collect diagnostic information (e.g., measure the level of glucose in the blood of diabetic patients) and to fine-tune medical treatments (e.g., adaptively regulate the dose of insulin administered) over extended periods of time.
One obstacle of enabling this vision of networked implantable devices is posed by the physical nature of propagation in the human body, which is composed primarily (65%) of water, a medium through which RF electromagnetic waves hardly propagate, even at relatively low frequencies. Most research in body area networks to date has focused on traditional RF communications along the body surface. Accordingly, most research has focused on reducing the radiated power to avoid overheating of tissues. The core challenge of enabling networked intra-body communications through body tissues is substantially unaddressed.
Acoustic waves, typically generated through piezoelectric materials, are known to propagate better than their RF counterparts in media composed mainly of water. Since World War II, piezoelectrically generated acoustic waves have found application, among others, in underwater communications (typically at frequencies between 0 and 100 kHz), in indoor localization in sensor networks, and, massively, in ultrasonic medical imaging.
While communication at low frequencies requires sizable transducers, innovations in piezoelectric materials and fabrication methods, primarily driven by the need for resolution in medical imaging, have made micro and nano scale transducers a reality. Moreover, the medical experience of the last decades has demonstrated that ultrasound is fundamentally safe, as long as acoustic power dissipation in tissue is limited to specific safety levels. Ultrasonic wave heat dissipation in tissues is minimal compared to RF waves.
A major challenge to using ultrasonic waves for communication is the temporal and spatial uncertainty that characterizes ultrasonic interference generated by devices in the human body and denser deployment in specific areas (e.g., heart or brain). The temporal uncertainty of interference is caused by asynchronous transmissions of different devices and time-varying ultrasonic communication channels. Moreover, the distance-dependent propagation delay of ultrasonic signals generates spatial uncertainty in interference. For example, acoustic signals simultaneously transmitted by different nodes located at different distances from an intended receiver, or different multipath replicas of the same signal, do not necessarily reach the receiver at the same time.
Cross-layer optimization algorithms for wireless networks exist. However, algorithms designed for RF wireless communications are not designed for the spatially and temporally uncertain ultrasonic environment. In addition, these RF algorithms often require coordination and message exchanges that are not desirable in this environment.
Recent efforts have attempted to address some of the challenges of interference modeling at the MAC layer. For example, with slotted transmission the packet collision probability can be reduced by adding a guard band to each time slot to limit the effect of the spatial uncertainty of interference. However, these solutions mainly rely on signaling exchanges that still suffer from the low-speed of sound, result in under-utilization of the channel, and therefore suffer from low throughput. Furthermore, these solutions look at the problem from a medium access control (MAC) perspective, exclusively. In addition, previous solutions are largely based on the protocol interference model (i.e., a packet is lost whenever two transmissions overlap at a receiver) which is not the case with ultrasonic intra-body transmission schemes. Ultimately, existing models fail to capture the statistical behavior of time-varying and spatially uncertain ultrasonic channels.